Multiplex Section Protection (MSP), which is a protection switching technology in transport networks and defined in G.841 of the International Telecommunication Union (ITU), has been widely applied in transport networks currently.
In 1:n MSP, 1+n paths are provided between two network nodes, wherein one is a protection path and the other n paths are working paths. A series of protection switching requests are specified in G.841, e.g. Signal Fail (SF), Signal Degrade (SD), Wait To Restore (WTR), No Request (NR) etc., and different priorities are specified for different protection switching requests by G.841. The priories of the four protection switching requests above are in decreasing order. SF has the highest priority and NR has the lowest priority.
After receiving a remote protection switching request, a node compares a local protection switching request with the remote protection switching request first, outputs a protection switching request with a higher priority, runs operations of a state machine according to an outputted protection switching request with the highest priority subsequently, sends K bytes and performs a protection switching action, and etc.
FIG. 1 shows a schematic diagram of 1:1 MSP, one working path and one protection path are provided between node S1 and node S2. When the working path and the protection path are normal, both node S1 and node S2 will select the working path to send and receive flows. When a fault is detected at a working path from node S1 to node S2, as shown in FIG. 2, node S2 detects SF of a local working path, generates an a protection switching request for the SF of the local working path, and node S2 does not have other protection switching requests, thus SF of the local working path is a protection switching request with the highest priority. Since a remote protection switching request of node S2 is NR, which has a lower priority than SF of the local working path, the protection switching request with the highest priority of node S2 is SF of the local working path. Node S2 runs operations of a state machine, sends K bytes on the protection path and switches to the protection path to receive and send flows, wherein the K bytes include the protection switching request for SF of the working path of node S2 and information switched to the protection path. After receiving the K bytes on the protection path, node S1 extracts the protection switching request therein. The local protection switching request with the highest priority of node S1 is NR whose priority is lower than that of the remote protection switching request, therefore, the protection switching request with the highest priority of node S1 is SF of the working path. Node S1 runs operations of the state machine, and according to the received K-byte information switched to the protection path, switches to the protection path to receive and send flows. In addition, node S1 further needs to send K bytes on the protection path, wherein a K-byte protection switching request is a Reverse Request (RR), and information switched to the protection path, wherein the RR is used to acknowledge that a protection switching request from an opposite node is received.
After node S1 to Node S2 finish the switching, flows are sent and received on the protection path. When the working path from node S1 to node S2 is recovered from fault, node S2 detects SF removal of the local working path and generates a protection switching request for the SF removal of the local working path, wherein the request is also a local protection switching request with the highest priority. Since the remote protection switching request received by node S2 is an RR, the RR will not be involved in comparison of the priorities of protection switching requests according to definitions of G.841, therefore, the protection switching request with the highest priority of node S2 is the SF removal of the local working path. Node S2 runs operations of the state machine and enters a WTR state. After the WTR state ends, node S2 performs a switch-back operation, and sends K bytes to notify node S1 to also perform switch-back. After both node S1 and node S2 perform the switch-back, flows are sent and received on the working path.
G.841 protocol solve problems of 1:n MSP perfectly. However, G.841 protocol has some defects because actual networks are relatively complicated, especially in scenarios such as simultaneous restoration of failed links in the two directions of a working path between nodes at two ends.
As shown in FIG. 3, when all links in the two directions of the working path between node S1 and node S2 are failed, remote protection switching requests received by node S1 and node S2 are protection switching requests for SF of the working path. When the links in the two directions are restored simultaneously, node S1 and node S2 detect SF removal of the local working path simultaneously. When a local protection switching request is compared with a remote protection switching request, since the remote protection switching request is SF of the working path, whose priority is higher than that of the local protection switching request, both node S1 and node S2 fail to respond to a protection switching request for the SF removal of the working path. Node S1 and node S2 respond to the remote protection switching request instead and will send an RR request to the opposite node. After receiving the RR request of the opposite node, node S1 and node S2 will not enter the WTR state according to the state machine. Therefore, the nodes at the two ends can neither enter the WTR state according to a normal process, nor switch back to the working path normally to receive and send flows.
Therefore, G.841 protocol has the disadvantage that switch-back cannot be performed in scenarios such as simultaneous restoration of failed links in two directions of the working path between the nodes at the two ends, and there is no solution currently.